Major deposits of oil shale formations exist in the Western part of the United States. These formations contain a substance called kerogen which can be converted into a petroleum-like substance by application of heat. It is estimated that this reserved energy is larger than any other petroleum deposit. However, despite intensive efforts to devise economic techniques for shale oil extraction, this product is still not competitive with expensive crude oil.
One approach for extracting shale oil uses conventional mining techniques to remove large blocks of oil shale to the surface to be broken up into much smaller pieces for heat processing in above-ground retorts. This is a high-cost approach and produces undesirable effects, e.g., the disposal of expended oil shale.
Another approach which shows promise in reducing costs, with less environmental problems, processes the oil shale underground--in-situ oil shale processing. In this approach, two horizontal rooms, or tunnels, are bored one above and the other at the bottom of the portion of the deposit which is to be processed. Vertical, parallel holes, in which explosives are loaded, are drilled between the rooms. The explosives are detonated to effect "rubblization. The necessary heat for oil extraction from the oil shale rubble is provided by igniting the oil shale at the top of the rubble and burning the carbonaceous part of the oil shale in the presence of air which is pumped into the burning area. The shale oil extracted from the rock, in the presence of hot gas, drains down into the lower room from where it is pumped to the surface. A detailed description of in-situ oil shale processing is available in the references.
One key parameter which controls the successfulness of in-situ oil shale processing is the porosity in the oil shale rubble. The interstitial voids between the oil shale fragments facilitate flow passages for hot gas and extracted oil. Experiences have shown that a minimum porosity of 15 to 20% is necessary for efficient oil extraction. Porosity is defined as the volume percentage of voids in the rubble. Insufficient porosity will drastically reduce the amount of oil produced per oil shale mass processed. Relative rubble size uniformity is another important parameter which affects oil extraction efficiency.
In practice, both due to economic and safety reasons, only a limited number of holes can be drilled into the oil shale between the rooms; therefore, the volume ratio of the total hole volume to that of surrounding oil shale is far less than 15%. When the holes are filled with explosives and detonated, most oil shale is frozen in position even though it may be fractured. Only a small part of the oil shale near the top and bottom rooms is blown loose, and therefore, the major part of the oil shale has very poor porosity.
A departure from the conventional blasting technique is proposed in the invention. Unique explosive loadings and firing patterns are used to achieve sequential fracturing and loosening of the oil shale. An oil shale rubble of better porosity results.